1. Field of the Invention
The present invention relates to magnetic recording media, consisting of a nonmagnetic substrate and at least one magnetizable layer which is firmly applied thereon and is based on a magnetic material dispersed in at least one polyureaurethane and a polyurethaneurea (meth)acrylate.
Increasingly high requirements are being set for magnetic recording media with regard to recording, playback and aging resistance. The binder is becoming more and more important with regard to meeting these requirements.
For example, in order to improve the magnetic properties, in particular the residual induction, a high packing density of the magnetic material in the layer is desirable, which results in a reduction in the binder content of the layer. Attempts have also been made to achieve an improved signal/noise ratio and a very narrow switching field distribution by using increasingly finely divided magnetic materials having a pronounced and extremely uniform acicular shape. These materials are furthermore very often surface-modified to reduce aging phenomena. As a result of such measures, both division of the pigments in the dispersing process and the achievement of good dispersion stability are made considerably more difficult. Moreover, the magnetic layers must be very flexible and have high elasticity and a high tensile strength. To avoid drops in output level, a reduction in the coefficiency of friction, and an increase in the abrasion resistance and wear resistance are increasingly also being required. Furthermore, this mechanical stability of the magnetic layer must also be ensured at high temperatures and high atmospheric humidity.
2. Description of the Related Art
It is known that magnetic layers which are subjected to severe mechanical stresses contain polyurethane elastomers which prove to be advantageous binders. Polyesterurethanes as described in DE-B 1 106 959, DE-B 2 753 694, EP-A 0 069 955 or U.S. Pat. No. 2,899,411 have proven particularly useful.
However, these binders can no longer adequately fulfil all the measures and requirements described above. In many cases, the pigment wetting and dispersing, particularly in the case of the very finely divided materials, are adversely affected so that any sintered components are divided up only to an insufficient extent in the milling process or agglomeration of the pigment particles is not adequately prevented, resulting in poor magnetic properties and consequently in poor tape properties in terms of electroacoustics and video data.
Low molecular weight dispersants are therefore added to facilitate the dispersing process. However, these dispersants have disadvantages. Thus, under unfavorable conditions, such as high temperature and/or high atmospheric humidity, low molecular weight dispersants may be readily exuded. In recording or playback apparatuses, this results in deposits on all parts in contact with the tape, particularly on the head, resulting in drops in output level. Moreover, the friction (adhesion) increases greatly, and this may result in the tape coming to a stop, ie. blocking. Furthermore, low molecular weight dispersants have strongly polar groups, and this has an adverse effect in the presence of water/moisture. Thus, the layer is rendered strongly hydrophilic by the presence of polar ionic groups, resulting in strong water absorption by the layer. Consequently, on the one hand the mechanical properties of the layer are adversely affected and on the other hand the pH of the layer is shifted considerably into the acidic range in the case of, for example, anionic groups, such as sulfonate or phosphonate groups, with the consequence of possible hydrolytic degradation of binders or the danger of pigment corrosion.
On the other hand, when conventional dispersing resins are used, there may be compatibility problems in the dispersion. Since these dispersions likewise contain polar groups, the hydrophilicity of the layer increases considerably and hence, especially in a humid climate, effects such as swelling of the layer, exudation of the dispersants and lubricants and changes in the mechanical properties due to changes in the plasticizer effects also increase substantially.
To improve the dispersion properties of polyurethane binders, it was proposed at an early stage to incorporate polar groups into the binder. These polar groups can in principle be introduced via any component which is used in the preparation of the polyurethane. Polyesters having polar groups are most frequently used (cf. inter alia DE-A 28 33 845). The incorporation of diols which additionally carry polar groups is described, for example, in JP-A 57 092 421, DE-A 38 14 536 or EP-A 193 084. The subsequent incorporation of the polar groups by an S.sub.N reaction at the terminal OH groups of the polyurethanes is disclosed in JP-A 57 092 422. These dispersing resins divide up the pigments very well on the one hand but on the other hand, especially with increasing fineness of the particles of the magnetic pigments used, lead to dispersions having extremely disadvantageous rheological properties, such as a high flow limit and high viscosity, so that further processing of these dispersions to magnetic layers entails considerable difficulties. Furthermore, the content of ionic dispersant groups in the dispersing resin is limited to a maximum concentration, since otherwise adverse effects, for example bridging flocculation, occur. An increase in the pigment volume concentration on the one hand and a reduction in the particle size on the other hand are then no longer in conformity with the required content of dispersing resins.
An improvement in the dispersing behavior could be achieved by low molecular weight, OH-containing polyurethanes, as described in EP 0 099 533. Substantial improvements in the dispersing behavior were achieved with polyurethaneurea (meth)acrylates, as described in German Patent Application P 4141838.7. However, these measures too are insufficient for dispersing the increasingly more finely divided, and surface-modified pigments and for meeting the constantly growing requirements which magnetic recording media have to meet with regard to mechanical and magnetic properties.
The use of trialkoxysilanes for dispersing pigments whose surfaces have been treated with SiO.sub.2 and Al.sub.2 O.sub.3 is known (cf. inter alia U.S. Pat. No. 4,034,139 and DE-A 33 30 380). Since these trialkoxysilanes are sensitive to hydrolysis, they could be handled and used only in the absence of OH groups, since the basic principle of the mode of action was based on the reaction between the trialkoxysilane and the OH groups of the pigment surface.